1. Field of the Invention
The invention relates generally to pumps and motors that have a rotor rotatably disposed within a stator. More particularly, the invention relates to reinforced stators for pumps and motors and methods of fabricating the same.
2. Background of the Technology
A progressive cavity pump (PC pump), also know as a “Moineau” pump, transfers fluid by means of a sequence of discrete cavities that move through the pump as a rotor is turned within a stator. Transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor relative to the stator, as well as relatively low levels of shearing applied to the fluid. Consequently, progressive cavity pumps are typically used in fluid metering and pumping of viscous or shear sensitive fluids, particularly in downhole operations for the recovery of hydrocarbons. A PC pump may be operated in reverse and functioned as a positive displacement motor (PD motor) to convert the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling.
As shown in FIGS. 1 and 2, a conventional PC pump 10 includes a helical-shaped rotor 30, typically made of steel that may be chrome-plated or coated for wear and corrosion resistance, disposed within a stator 20, typically a heat-treated steel tube or housing 25 lined with an elastomeric stator insert 21 having a helical-shaped through bore. Specifically, stator insert 21 is bonded to the inner surface of stator housing 25 with a bonding agent such that insert 21 does not move relative to housing 25. Alternatively, for thin-walled liners, the stator housing has a helical-shaped inner surface and the stator liner disposed within the housing has a uniform radial thickness elastomeric layer or coating bonded to the inner surface of the stator housing.
The helical-shaped rotor 30 defines a set of rotor lobes 37 that intermesh via interference fit with a set of stator lobes 27 defined by the helical-shaped insert 21. As best shown in FIG. 2, the rotor 30 typically has one fewer lobe 37 than the stator 20. When the rotor 30 and the stator 20 are assembled, a series of cavities 40 are formed between the radially outer surface 33 of the rotor 30 and the radially inner surface 23 of the stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along contact lines between the rotor 30 and the stator 20. The central axis 38 of the rotor 30 is parallel to and radially offset from the central axis 28 of the stator 20 by a fixed value known as the “eccentricity” of the PC pump.
During operation of the PC pump 10, the application of torque to rotor 30 causes rotor 30 to rotate within stator 20, resulting in fluid flow through the length of PC pump 10. In particular, circumferentially adjacent cavities 40 are opened and filled with fluid as rotor 30 rotates relative to stator 20. As this rotation and filling process repeats in a continuous manner, fluid flows progressively down the length of PC pump 10.
Since PC pumps and motors have few components, they can be made with a relatively small diameter sufficient for use in downhole applications. Another advantage of PC pumps and motors is that the fluid flowing through the PC pump or motor can contain some solid particles without risking damage to the pump or motor. For example, drilling mud that is used to cool and lubricate the drill bit and to raise cuttings to the surface may be used as the drive fluid for a PC motor.
Conventional PC pumps and motors can reach operating temperatures up to 300° C. or more depending on the ambient temperature and its operating efficiency. The operating temperature of a PC pump or motor is a function of various factors including frictional engagement between the stator and the rotor and cyclical deflections of the elastomeric lobes of the stator liner, which are due, at least in part, to the interference fit with the rotor and associated reactive torque. The cyclic deflections and frictional engagement of the rotor and stator are known to cause increases in the operating temperature of the stator insert. Although some of the thermal energy generated in the stator insert is carried off by the fluid medium flowing through the PC pump or motor (e.g., drilling mud), a substantially amount of thermal energy remains in the stator and can negatively impact performance of the PC pump or motor. In particular, most stator inserts are made from a synthetic elastomer or natural rubber compound that exhibits a relatively high coefficient of expansion. Thus, the thickness of the elastomeric stator insert can change considerably as a function of the operating temperature of the stator, which in turn, can alter the interference fit between the rotor and the stator. For example, the elastomeric material of the stator insert may expand to the extent that frictional losses due to engagement of the rotor and the stator begin to significantly reduce the efficiency of PC motor or pump. Even worse, in some cases, the elastomeric stator liner may expand to the extent that rotation of the rotor is completely inhibited. In addition, excessive frictional engagement between the rotor and the stator may strip away the liquid between the rotor and stator, potentially leading to a dry contact region and resulting damage to the elastomeric material.
Accordingly, there remains a need in the art for stator inserts for PC pumps and motors that offer the potential for improved thermal energy dissipation, reduced operating temperatures, and reduced frictional engagement with the rotor. Such improved stator inserts would be particularly well received if they were easily removed from the stator housing for service, maintenance, and replacement. Furthermore, it would desirable to spatially vary the radial thickness of the stator liner to minimize thermal degradation of the elastomer liner and spatially vary its stiffness to improve its performance.